The present invention relates generally to the railroad industry and, more particularly, to a railroad truck (commonly referred to as a "bogie" in many parts of the world) for supporting a railroad car.
A standard freight railroad car traditionally includes a truck at each end having two or more wheel sets to support the body of the railroad car. A conventional railroad freight truck is referred to as a three-piece truck design even though the truck has five or more major parts--a lateral bolster, two side frames and two wheel sets. (A wheel set is a rigid assembly of an axle, 2 wheels and bearings; each wheel is fixed to the axle and does not rotate independently.)
The structure of the traditional railroad truck permits operation on straight and curved track but has serious disadvantages. Trucks are able to negotiate both straight and curved sections of track by virtue of the tapered shape of each wheel and a traditional design that allows each wheel set axle to yaw in curves.
In more detail, depending on the point of contact between each wheel and the track, the wheel will have a different radius, and will traverse a different linear distance for each wheel rotation. On straight sections of track, each wheel of a wheel set ideally contacts the track at the same radius so that each wheel traverses the same linear distance for a given wheel rotation. On curved sections of track, the wheel contacting the rail on the outside of the curve contacts at a larger wheel radius, and traverses a larger linear distance than the wheel on the inside of the curve. These unequal wheel radii ideally cause the axle to yaw and thereby steer into the curve. Such wheel set yaw necessarily requires a non-rigid mounting between each wheel set and the truck.
The traditional three-piece railroad truck design provides this non-rigid mounting. Each side frame is aligned approximately parallel to a track and attaches to one end of each wheel set. The bolster is a cross-member which spans each side frame and is attached to the car body so as to provide for rotation of the truck relative to the car body in curves. The non-rigid attachment of each side frame to the bolster provides for movement of the side frame, and of the wheel set axles coupled thereto, relative to the truck bolster. The non-rigid attachment has historically been provided by a means for absorbing and releasing energy, such as by springs and/or springs and dampers, for example.
While the use of tapered wheels and a non-rigid wheel set mounting configuration that allows wheel set yaw, permits operation on straight and curved track sections, the traditional design is disadvantageous.
A problem faced by this conventional design is that even on straight track, the tapered wheel design causes the wheels to follow a sinusoidal path having a wavelength of about 55 to 65 feet. As a result of an initial alignment offset or some perturbing force, each wheel of a wheel set pair may contact the rail at a different radius. The difference in wheel radii cause the axle to adjust to the unequal radii by yawing away from the wheel rolling on the larger radius. The yaw is constrained to some maximum amount by the mounting between the side frame and the bolster and when this maximum is reached, the direction of yaw is reversed. This results in the observe sinusoidal oscillation. The difference in wheel radii causes the traditional three-piece truck design to parallelogram, thereby creating rolling resistance and consequent wheel and track wear. The axle yaw acceleration increases with speed and may create lateral wheel forces sufficient for the wheel to climb the rail. Wheel axle yaw also creates lateral acceleration at frequencies that may be damaging to the railroad car structure or the lading.
In order to minimize the unfavorable effects of steering on straight track, the wheel set axle should be held rigid and perpendicular to the track. This goal may in principle be accomplished by increasing the turning moment and warp moment by increasing the stiffness and friction associated with the relative movements between the bolster and the car body and between the side frames and the bolster.
While the sub-optimal performance of the traditional three-piece truck design was tolerated in a less competitive transportation era, the availability of alternative transportation strategies has prompted the railroad industry to attempt to improve operating costs in the areas of improved fuel economy, lower maintenance, reduced lading damage, and better productivity, by running fewer cars faster, at higher capacity, and more often to achieve the same or higher annual tonnage.
However, an increase in stiffness and friction to improve straight track performance, contrarily reduces the effectiveness of steering on curved track sections by reducing the ability to yaw. The contrary requirements create a design impasse in the traditional three-piece truck.
Therefore, a problem faced by the railroad industry was how to simultaneously provide optimum or near-optimum operation on both straight and curved track. A major contributor to fuel consumption when an existing railroad truck is used, is rolling resistance. Rolling resistance also contributes to wheel and track wear, both being major components of maintenance cost. Lateral oscillations are a major contributor to lading damage. Finally, the existing railroad truck design also limits operating speeds to speeds established in the early 1900's.
The need for increased safety at any cost is enhanced by the recognized danger of transporting hazardous materials such as chemicals or nuclear materials.
There have been some attempts to solve the problems associated with the traditional truck, but these attempted solutions have by and large been technologically inadequate or economically infeasible.
So called self-steering radial trucks have been developed with a passive compliant connection between the wheel set axles to allow the axles to move radially on curves. This eliminates some of the problems associated with travel on curves but generally does not improve straight track performance and may actually create problems for travel on straight track so that steering on straight track becomes marginal. The designs require relatively unworn high conicity wheel trend profiles (e.g., 40:1) as commonly used outside the United States of America. In the United States of America wheel treads have a low conicity ratio (20:1). For low conicity profiles and for worn high conicity profiles, the use of passive compliant connection results in loose steering on straight track. These radial trucks have unstable motion characteristics (hunting) on straight track that leads to high friction and wear, as well as poor ride quality and the possibility of derailment.
Another attempt to improve performance involved cross linking a wheel from each wheel set on opposite sides of the truck to minimize parallelogramming of the axles and to minimize the hunting tendency. See, for example, U.S. Pat. No. 4,480,553 issued Nov. 6, 1984 to Scheffel. The cross linking of wheel sets links the wheel sets so that the natural steering forces generated by the differences in the wheel rolling radii at the rail contact point cause the trailing axle to urge the leading axle towards a radial position. This cross linking may effect some improvement for low speed freight, but generally cannot be applied at high speed because the hunting problem remains on straight track.
Another attempt to improve the performance is to couple truck wheel sets using solid arms and an elastomeric coupling material to allow relative movement between wheel sets. If such systems were designed as linear systems of elastomeric dampers for dominant frequencies, they may not provide the desired performance because the operational environment may be highly non-linear, particularly in light of truck, wheel and rail wear. See, for example, U.S. Pat. No. 4,781,124 issued Nov. 1, 1988 to List. However, this attempted solution does not provide both optimum straight and curved track performance.
Although prior art trucks may improve performance on curved track, they are not sufficient to satisfy other requirements. Each of these systems attempts to align each wheel set axle to the center of the curve. Unfortunately, because the conflicting requirements of maximum stiffness for straight track conflicts with the requirement for minimum stiffness for curved track operation, the solution is incomplete. Even if a passive steering system could be optimized for curves, its performance on straight track would be degraded.
Also, the steering force for a passive steering system is generated in response to the geometry of a single wheel set relative to the track, or at most to the geometry of the truck relative to the track. As such, the orientations of each wheel set of the truck or of the railroad car are uncoordinated.
There have been attempts to implement forced-steering systems which incorporate a linkage or flexure system to coordinate the movement of the two wheel set axles. See for example U.S. Pat. No. 4,295,428 issued Oct. 20, 1981 to Dickhart et al., and U.S. Pat. No. 3,789,770 issued February of 1974 to List. However, prior attempts to implement forced-steering have been inadequate.